1. Field of the Invention
This invention relates to high-power-and-narrow-linewidth single-frequency lasers and more specifically to the use of a pair of polarized single-mode pump lasers, which are driven below their respective “micro-kink” points, thereby reducing the laser's linewidth.
2. Description of the Related Art
Rare-earth doped glass lasers were first proposed in the 1960s and have received considerable attention in the 1980s for potential applications in optical communication. For laser emission to occur, the active medium is placed inside a resonant cavity. The optical feedback can be provided simply by the reflectivity of the end facets, by mirrors, by distributed feedback Bragg (DFB) gratings, or by distributed Bragg reflectors (DBR), or by constructing a ring cavity structure. Laser emission occurs when the total gain overcomes the losses in the cavity. Hence, a minimum gain has to be achieved to reach the laser threshold condition. Robust single mode (single wavelength) performance can be achieved using an ultra-short cavity of less than about 5 cm together with a wavelength selective reflector. The output power is dictated by the total absorbed pump power, which is generally proportional to the number of active ions and therefore proportional to the length of the cavity as well as to the crossectional area of the active material inside the cavity. While longer cavities lead to more stringent requirements on the longitudinal mode selector, a larger crossectional area typically sacrifices single transverse mode operation of the laser. Thus, output power must typically be traded off against single frequency and single-mode performance. The spectral linewidth of single frequency lasers, defined as the wavelength interval over which the magnitude of all spectral components is equal to or greater than a specified fraction of the magnitude of the component having the maximum value, is in general determined by a variety of noise contributions from the pump laser, the active medium itself, or the laser cavity.
For many applications such as fiber optic sensing, coherent optical communication, or as seed laser for laser ranging and LIDAR applications, high power (>10 mW and preferably greater than >25 mW), narrow linewidth (<10 KHz) single mode lasers that operate in the eyesafe spectral region of the telecommunication band around 1550 nm are in demand. These lasers include fiber, waveguide and microchip lasers. For example, (DFB fiber laser see J Lightwave Technology 16 114 (1998), waveguide laser see Applied physics Letters 74 789 (1999), Microchip laser see Electronics Letters 28 2067 (1992)).
Many of these narrow linewidth single-frequency fiber lasers are pumped with a pigtailed single-mode pump diode that excites the dopant ions to provide gain. The pump diodes have a P-I curve 10 (optical power vs. current) as shown in FIG. 1 that exhibits a pronounced kink 12 which is due to the occurrence of higher order transverse modes inside the semiconductor laser chip. The optical power at this point is commonly referred to as the “Kink Free Power”. The laser kink limits the usefulness of the laser to optical powers below the Kink Free Power. To maximize power without increasing the noise, the diodes are typically driven just below this kink point.
Deployment of fiber optic sensing will require compact low-cost continuous single-mode lasers that can deliver greater than 50 mW of output power with a narrow linewidth.